The effect of cold rolling and annealing on microstructure and tensile properties of the nanostructured Ni50Ti50 shape memory alloy

In this paper, the effect of cold rolling and post-deformation annealing on the microstructural evolution and mechanical properties of the Ni50Ti50 shape memory alloy was investigated. The cast ingots were fabricated by a copper boat vacuum induction melting technique. The homogenized specimens were then hot rolled and annealed to prepare the suitable initial microstructure. Thereafter, annealed specimens were subjected to cold rolling at room temperature with various thickness reductions up to 70%. Microstructural investigations showed that by increasing the cold deformation, a high density of dislocations is accumulated, leading gradually to nanocrystallization and amorphization. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. Results showed that the stress–strain curves of the cold rolled specimens exhibited plastic deformation of austenite without the stress plateau region. However, the stress plateau appeared in the stress–strain curves of the annealed specimens, whose stress level was increased with increasing thickness reduction.